In its simplest conceptual form (shown in FIG. 1), a BLDC motor consists of a permanent magnet 1 (the rotor) which is free to rotate around its axis of symmetry, surrounded by an arrangement of at least three fixed electromagnets (the stator) consisting of solenoid windings 2, positioned at 120° relative to each other around the rotor axis. Each solenoid is energized by applying to it a DC (Direct Current) voltage, by means of a set of electronic switches 3, operated with timing and polarity determined by a switch-control algorithm.
If the electromagnets are energized with the proper timing and polarity, they generate a magnetic field with the proper strength and direction relative to the S-N axis direction of rotor magnet 1, and this magnetic field produces a torque on the permanent magnet causing the rotor to turn. The algorithm determines the required operating sequence of the switches at any given moment, according to the actual angular position of the rotor, said position being determined by means of one or more sensors, usually of the Hall type (indicated in the figure by numeral 4), which sense the magnetic field of the rotor. The operation of the motor, which is housed in a housing 5, is controlled by a controller 6.
In the simple conceptual form of FIG. 1, it is enough to properly energize two magnets at a time in order to generate a rotating magnetic field of arbitrary direction that will keep the rotor turning. In practice, in order to obtain a continuous smooth torque value, BLDC motors are implemented using many windings for the stator, and several magnets with alternating N-S poles for the rotor.
There are two basic BLDC motor architectures known in the art: the inner rotor architecture (FIG. 2a), where the stator windings surround the rotor and are affixed to the motor's housing, and the outer rotor architecture (FIG. 2b), where the stator solenoids are affixed in the core of the motor, and are surrounded by the rotor magnets. In the prior art implementations, BLDC motors suffer from the following drawback: for a fixed supply voltage, as the motor speed increases there is a decrease in the torque that the motor can provide. This undesirable effect is the result of the generation of a parasitic voltage, known as the back EMF (Electromotive Force) voltage.
The back EMF is a voltage generated in the stator much in the same way an electric generator works, because there is relative motion between the solenoids of the stator and the magnetic field created by the permanent magnets of the rotor. The magnetic field lines created by the permanent magnets rotate along with the rotor. Thus, the projection (in the direction of the solenoid axis) of the magnetic field lines entering the cross-sectional area of each of the energized solenoids, changes with time. This projection of field lines adds up to a quantity referred to as “the magnetic flux” through the solenoid. By Lenz's law of induction, a changing magnetic flux produces an induced voltage in the solenoids (in this respect, the motor acts like a generator). The value of this induced voltage increases proportionally to the rate by which the flux changes, and therefore it increases with increasing rotating speed of the motor, and its polarity opposes the original voltage externally applied by the supply. As a result, the overall effective voltage applied to each energized solenoid of the stator decreases with increasing angular velocity of the rotor (the overall voltage equals the constant external supply voltage, reduced by the induced back EMF. Due to the decrease in the overall voltage applied, the current flowing into the solenoids of the stator decreases too, which ultimately results in a reduction of the torque provided by the motor. Therefore, the maximal torque that the motor can deliver drops as the rotating speed increases. In order to increase torque at high speed, one needs to increase the supply voltage, an operation which in many instances cannot be done.
Another adverse side effect of back EMF generation is that, for a fixed supply voltage, the current flowing in the solenoids is higher at lower rotational speed, because then the back EMF is lower and the overall voltage applied to the solenoids is higher. It follows that at start (when there is no motion, and therefore there is no flux change and no back EMF) the motor drives the highest current. Since the supply voltage is significantly higher than the overall voltage applied to the solenoids at final speed, then, at motion start one gets peaks of current that are significantly higher than the steady-state working current. Such undesirable over-current peaks may even lead to solenoid damage or power supply overload, and sometime must be dealt with, by means of added protective devices, or by an overkilled design of current handling capability.